Add time_to_live parameter into the barclamp

Add the parameter time_to_live (ceilometer.conf) into the barclamp,
so the user can decide when the meter lines are removed from the
database.

This patch was tested with a mongodb configuration.

This patch was made by Jiri (jsuchome). I only tested it and added
the parameter in the ceilometer.conf.erb template.
Not sure it makes sense to say "The default is foo" as it won't matter as soon as it's changed. Maybe just say: "-1 means that samples are kept in the database forever" ?